Various decorating compositions containing gold sulforesinates have been applied and fired onto glasses and glazed ceramics to obtain this, bright specular films. Such material is described, for instance, in U.S. Pat. Nos. 2,984,575 and 3,163,665.
In these patents, gold sulforesinates obtained by reacting a gold salt with a sulfurized organic moeity, such as a terpene hydrocarbon or an organic mercaptan, are dissolved in an organic solvent, such as an essential oil, exemplified by lavender or rosemary, and mixed with a suitable flux, such as a salt of bismuth, chromium, tin, vanadium, copper, titanium or zirconium. The resulting decorating compositions are found capable of forming a bright thin film of metallic bright gold when applied and fired onto glass or glazed ceramics at temperatures above about 450.degree. C. The addition of small amounts of promoting agents and dopants to enhance the specularity and continuity of the resulting gold film is also permitted.
Where more elegant and expensive decorations are required, the gold decorating composition includes fine gold powders or flake, either with or without the gold sulforesinate component, along with an appropriate flux to obtain a substantially thicker, but dull fired film, which must be polished or burnished with wet sand or a dry fiberglass brush to obtain the desired satin luster. The latter type of decoration is generally termed "burnished gold," because of the polishing step. The satin luster and texture of burnished gold films, though elegant, are quite different from the much thinner, smooth, specular, bright gold films normally obtained by firing gold sulforesinates alone.
Both types of gold films have widespread appeal and utility, however, the sometimes tedious extra polishing required in the latter technique causes a thinning and weakening of the precious metal coating. In such case, redecorating, refiring and reburnishing may be needed to repair and salvage the product. Such, of course, is costly. In addition, the very fine particles resulting from polishing can become airborne and present a potential health hazard to the operator.
It is an object of the present invention to obtain a precious metal composition, such as gold, platinum or silver, which can be applied easily and conventionally on ceramic and glass substrates in the manner of bright gold coatings, but which, when fired, will provide a lustrous, adherent coating of the type and appearance of the expensive burnished metal without the above disadvantages and cost.